Temperature stabilized cavity resonator



Nov. 25, 1969 A. KACH 3,480,889

TEMPERATURE STABILIZED CAVITY RESONATOR Filed July 13, 1967 Tuzll 4 9 INVENTO .44 F250 AGI/ BY f ez. RAT l ATTORNEY United States Patent US. Cl. 333-83 8 Claims ABSTRACT OF THE DISCLOSURE In a microwave cavity resonator being comprised of a metallic casing enclosing a resonant cavity, the variations of the resonant frequency due to thermal expansion and contraction of said casing as a result of temperature fluctuations are compensated by the provision of a split bi-metallic'sleeve coaxial with and enclosing the inner end portion of a tuning plunger, both said sleeve and plunger being connected to and projecting from opposite end walls of said casing and into said cavity. The plunger cooperates with the bi-metallic strips of the sleeve, to form a reentrant or lumped capacity of the cavity determining its resonant frequency. Besides, the bi-metallic strips, produced for instance by simple longitudinal slitting of the end of a bimetallic cylinder, are so designed as to cause the same to be deflected away from the plunger with increasing temperature, to reduce said capacity and to in turn increase the resonant frequency of the cavity, and to be deflected towards the plunger with decreasing temperature, to increase said capacity and to in turn decrease the resonant frequency, in such a manner as to counteract the increase and decrease, respectively, of the resonant frequency fluctuations resulting from thermal expansion and contraction of the cavity. By providing for relative axial adjustment of both the sleeve and plunger, the effect of thermal volume changes of the cavity on and other causes affecting the resonant frequency may be substantially compensated by the stabilizing means according to the invention.

The present invention relates to high-frequency cavity resonators for use in microwave systems, more particularly to resonators embodying a tuning plug or plunger for the control of the resonating frequency of the cavity, a major object of the invention being the provision of simple and eflicient stabilizing means to compensate for the effect of temperature fluctuations and other causes affecting the resonant frequency of resonators of this type.

The effect of temperature on the resonant frequency of a cavity resonator is partly determined by the coeflicient of thermal expansion and contraction of the material of the casing enclosing the resonating space or cavity. As an example, the frequency shift for brass as material per degree Centigrade is found to be about 0.02 percent of the prevailing resonant frequency. This relatively large amount of drift of the resonance point may be reduced by the use of a material having a lower coeflicient of expansion than brass, such for example as an iron-nickel alloy known under the name of INVAR which has a very small coeflicient of thermal expansion. This, however, aside from other drawbacks, leads to relatively high costs for both material and production.

In order to compensate for the effect on the resonant frequency caused by thermal expansion and contraction of a cavity resonator, it has been proposed to connect a tuning plunger with the resonator casing through an intermediate telescopic member made of parts having dif- 3,480,889 Patented Nov. 25, 1969 ferent coefficients of expansion, whereby a temperature change results in additional displacements being imparted to said plunger, to counteract the resonance shift of the resonator cavity caused by its thermal expansion and contraction, respectively. Practical experience has shown, however, that, among other shortcomings and drawbacks, such a solution results in both bulky and impractical devices and structures.

Aside from being determined by the dimensions of the casing, the resonance frequency of a cavity resonator may be furthermore affected by the reactalnces of the input and output coupling circuits of the resonant cavity con sisting of or containing non-linear circuit elements or reactances. In such a case, reactance variations caused by temperature fluctuations may result in additional contribution to the resonance drift of a cavity resonator. This is found to be especially the case with harmonic frequency multipliers embodying a variable capacitancediode or varactor in its coupling circuit as a harmonic generator. In fact, variable capacitance diodes exhibit a relatively large increase in capacity with rising temperature, whereby to result in a substantial displacement of the resonance point of a tuned circuit or cavity coupled with such diodes.

In order to compensate for the foregoing temperaturedependent reaction by a capacitance-diode, suppliers recommend the insertion of a temperature-dependent resistor or thermistor in the reverse D.C. biasing circuit of the diodes, said resistor having a large enough positive temperature coefficient as to increase the DC. inverse biasing voltage of the diodes with rising temperature, and vice versa, to an extent as to maintain a substantially constant effective capacity at any particular resonance frequency. This type of compensation is restricted, however, to multiplier circuits of specific design, thus greatly reducing its usefullness in practice.

Accordingly, among the further objects of the present invention is the provision of new and improved temperature-stabilizing means for a cavity resonator by which the prior and related difficulties and drawbacks are substantially overcome; which is both simple in design and efficient in operation; which will allow the degree of compensation to be set or adjusted simply and reliably; which is capable of simultaneously compensating for the effects on the resonant frequency of a cavity resonator caused by thermal expansion and contraction of the resonator casing or cavity, as well as by the reaction upon the resonator from the input and output circuits coupled therewith, and which can be applied to existing resonator structures readily and at relatively low cost.

The invention, both as to the foregoing and ancillary objects as well as novel aspects thereof, will be better understood from the following description, taken in reference to the accompanying drawing forming part of this specification and in which:

FIG. 1 is a cross-section of a cylindrical cavity resonator structure embodying improved frequency-stabilizing means constructed according to the principles of the invention;

FIG. 2 is an enlarged and fractional section taken on line 22 of FIG. 1; and

FIG. 3 is a section similar to FIG. 1, showing a stabilized cavity resonator constructed according to the invention embodied in a harmonic frequency multiplier.

With the foregoing objects in view, the improved cavity resonator according to the present invention is characterized essentially by the provision of a bi-metallic sleeve projecting into the resonator cavity from an end wall of the resonator casing and enclosing the end of a conventional tuning plug or plunger also projecting into said cavity from a wall opposite to said first Wall. As a consequence, said sleeve and plunger form a lumped reentrant capacity determinative of the resonance frequency of the cavity, the latter being otherwise controlled by the cavity geometry and dimensions. In order to effect compensating capacity and in turn frequency variations by said lumped capacity, to counteract the frequency variations caused by thermal expansion and contraction of the resonator casing, the free end of said sleeve is subdivided by longitudinal slits into a plurality of bi-metallic strips or reeds, the construction of said reeds being such as to become bent radially outwardly or away from the plunger as the temperature increases, and to be bent radially inwardly or towards the plunger as the temperature decreases, in such a manner as to effect compensation of the resonant frequency changes of the cavity due to thermal expansion and contraction, as will become more apparent as the description proceeds in reference to the drawing, wherein like reference numerals denote like parts in the different views thereof.

Referring more particularly to FIG. 1, there is shown a basic form of a temperature-stabilized cavity resonator according to the invention, comprising a cylindrical metal casing 1 enclosing a resonant cavity and including means, such as a pair of coupling loops 2 and 3, to feed highfrequency or microwave energy to extract it from the resonator, such as via input and output coaxial feeders 4 and 5, respectively, mounted in the cylindrical wall of the casing in accordance with conventional and wellknown practice. There is furthermore provided a tuning plug or plunger 6 threadly adjustably mounted in one of the end walls of the casing, that is, the bottom wall according to the example, as more clearly shown at 60.

The plunger 6, projecting to a varying distance into the casing, converts the latter into a so-called reentrant cavity with the end surface of the plunger forming a lumped capacity together with the opposite or upper end wall, in the example shown, of the casing. As a consequence, adjustment of the plunger results in a variation of the reentrant capacity, forming a capacitative load on the resonator, and in turn of the resonant frequency of the cavity, the latter being otherwise a function of the geometry and dimensions of the casing, in a manner well known to those skilled in the art. As can be seen, expansion and contraction of the casing 1 due to ambient or internal temperature fluctuations will result in a decrease and an increase of the resonant frequency in proportion to the temperature increase or decrease, respectively.

In order to compensate for the variations of the resonant frequency caused by the thermal expansions and contractions of the cavity, in accordance with the improvement of the present invention, the tuning plunger 6 projects into a bi-metallic sleeve 7 mechanically and electrically connected to the upper end wall of the casing, in the example illustrated, and also projecting into the cavity. As a consequence, the reentrant or lumped capacity of the cavity is formed by the end portion of the plunger 6 and the encircling portion of the sleeve 7. In order to vary this capacity in the proper sense and, in turn, the resonant frequency of the cavity so as to compensate for the changes due to thermal expansion and contraction of the casing, at least the end portion of the sleeve 7 is formed with a plurality of spaced longitudinal slits 8, to provide a plurality of bi-metallic reeds or strips 9 enclosing the plunger 6, as more clearly shown in FIG. 2, wherein the bi-metallic nature of the strips 9 is indicated by the dotted median lines in the drawing.

As a consequence, the tuning plunger 6 and bi-metallic strips or reeds 9 constitute a pronounced temperaturedependent capacitative load on the cavity resonator, the capacity being a function of the distance d between the reeds and the plunger, in a manner readily understood. The strips 9, being preferably produced by cutting or slitting a bi-metallic tube or cylinder, are so designed as to be deflected inwardly or toward the plunger as the temperature decreases, as indicated at in FIG. 2, to thereby increase the reentrant capacity of the cavity, and to be deflected outwardly or away from said plunger as the temperature increases, as indicated at 9b in the drawing, to thereby decrease the reentrant capacity, respectively. As a consequence, by the proper design or adjustment of the mean capacity between the plunger 6 and the reeds or strips 9, the increase of the resonant frequency by the reeds moving outwardly during a temperature increase substantially compensates the decrease of the resonant frequency due to expansion of the casing 1, and vice versa. Adjustment of the average reentrant capacity may be effected by control of the overlap of the plunger by the reeds 9 by adjustably mounting the sleeve 7, as shown in FIG. 3 described in the following.

FIG. 3 shows, by way of example, a temperaturestabilized resonant cavity according to the invention embodied in a variable capacitance-diode frequency multiplier or harmonic generator. The input cavity 16 of the multiplier being resonant to the fundamental frequency and comprised of elements 14 and 69 is substantially similar to FIG. 1 and is excited or energized by highfrequency energy at fundamental frequency from a suitable source (not shown) via the coaxial feeder 4. The bi-metallic sleeve 7 is shown mounted upon a separate adjustable screw or threaded bush 10, to make it possible to remove the sleeve for prebending reeds 9 and trimming or adjustment of the mean reentrant capacity of the cavity 16. This advantageous possibility of fitting or removal of the sleeve 7 is desirable for the purpose of conveniently adjusting the capacity by varying the length of the reeds, while axial adjustment provides a further possibility of tuning by varying the reentrant capacity of the cavity.

The high-frequency or microwave energy of fundamental frequency developed within the cavity 16 is transmitted, via the output coupling loop 3, to the output cavity 15 of the frequency multiplier, resonant to the desired harmonic of said fundamental frequency, through a space coupling arrangement of the type shown and described in the present applicants US. Patent No. 3,278,868 and including a variable capacitance-diode or the like non-linear distorting element adapted to generate harmonics of said fundamental frequency, one of which is selected or segregated by the output cavity 15 of the multiplier resonant to the desired or useful harmonic frequency, in a manner customery with harmonic frequency generators or multipliers of this type.

For the latter purpose, the inner conductor or feeder 11 connected to the coupling loop of cavity 16 and forming a coaxial line in conjunction with the casing, serially includes a negatively biased variable capacitance junction diode or varactor 12 removably mounted in the casing wall by means of a threaded bush or screw 13. The coaxial feeder including the diode 12 terminates in the nich-like auxiliary cavity 14 communicating with the output cavity 15, to enable the ready flow or coupling of the high-frequency energy, the degree of coupling or matching of the circuits being adjustable by means of a further tuning plunger 17 operatively associated with the cavity 14, in the manner described in greater detail in applicants aforementioned prior patent.

The additional tuning plunger 18 of the cavity 16 serves for matching purposes together with the plunger 6 in any particular case to provide a resonance match for the multiplier and to enable it to deliver a maximum of output power. Item 19 is an ordinary tuning plunger and 20 an output coupling loop for the cavity 15, to deliver microwave power at the desired harmonic frequency to an output or utilization device via the coaxial feeder 21.

The invention when used in connection with a temperature-dependent capacitative load feed by the cavity resonator, such as the varactor diode 12 in the case of FIG. 3, has the further advantage of compensating for the varying temperature-dependent reaction of the load on the resonant frequency of the cavity in substantially the same manner as and in addition to the compensation of the direct resonance variations due to thermal expansion and contraction of the cavity. Thus, in the example of the varactor load 12, the effect of temperature and resultant change of the resonant frequency of the cavity 16 also alters the capacity of the diode 12 which in turn reacts upon the resonator, to cause additional temperturedependent displacements of the resonance point of the cavity. As pointed out, as the temperature increases, the capacity of the varactor diode and its reaction on the cavity also increases, and vice versa.

More particularly, assuming first a given operating frequency of the resonator, an increase of the capacity of the diode 12, in response to increasing temperature and, in turn, increased reaction upon the resonator and lowering of the resonant frequency of the cavity 16, may be compensated by outward bending of the reeds 9, in sub stantially the same manner and in addition to the compensation in response to expansions of the cavity caused by a temperature increase, and vice versa, provided a proper adjustment of the mean reentrant capacity.

Besides, the invention also acts to take ready care of the varying degrees of reaction of the varactor 12 upon the cavity 16 for different frequencies within the operating range of the tuning plunger 6. Since the mean capacity of the diode 12 remains practically constant over the entire tuning frequency range of the resonator, the reaction at the higher frequencies or within the upper part of said range is greater than at the lower frequencies or Within the lower part of said range, due to the decreased tuning capacity and tighter coupling, and vice versa. This behavior corresponds to the variation of the relative capacitative load changes of the resonant cavity caused by the temperature-dependent deflections of the reeds 9 of the bimetallic sleeve 7. More particularly, the relative variation of the capacity between the tuning plunger 6 and bi-metallic reeds 9 is determined by the mean deflection of those regions of the reeds which overlie the plunger 6, said mean deflection increasing as the extent to which the plunger penetrates into the sleeve, that is, as the resonant frequency of the cavity is increased and vice versa.

As has been found by experiments, a proper choice of the thickness and other characteristics of the bi-metallic strips or reeds and proper adjustment or trimming thereof, as well as the axial position of the sleeve, will result in an almost complete compensation for the effect of temperature fluctuations on the resonant frequency of a cavity resonator over a relatively wide tuning range.

In the foregoing the invention has been described in reference to a specific and exemplary device. It will be evident, however, that variations and modifications, as well as the substitution of equivalent parts or elements for those shown herein for illustration, may be made without departing from the broader scope and spirit of the invention as set forth in the appended claims.

I claim:

1. A temperature-stabilized cavity resonator comprising in combination:

( 1) a closed metallic casing providing a resonant cavity,

(2) a tuning plunger mounted in said casing and projecting into said cavity, to determine its resonant frequency, and

(3) a plurality of spaced and parallel bi-metallic strips arranged along the circumference of a circle and connected to form a split sleeve having one end connected to said casing with said strips concentrically enclosing the end portion of said plunger .in predetermined spaced relation thereto,

(4) said strips being designed to be deflected away from said plunger with increasing temperature and to be deflected towards said plunger with. decreasing temperature, respectively, of said casing.

2. A cavity resonator as claimed in claim 1, said casing having a cylindrical shape with said plunger and sleeve mounted coaxially in the opposite end walls of the casing.

3. A cavity resonator as claimed in claim 1, said plunger being adjustably mounted in a first end wall of said casing and said bi-metallic sleeve being secured in a second end wall of said casing coaxially with said plunger.

4. A cavity resonator as claimed in claim 1, including further means to displace said sleeve axially in respect to said plunger.

5. A cavity resonator as claimed in claim 4, said last means including a threaded bush removably mounted in said casing and carrying said sleeve.

6. A cavity resonator as claimed in claim 1, said sleeve consisting of a split bi-metallic cylinder coaxial with said plunger and electrically and mechanically connected to said casing.

7. A cavity resonator as claimed in claim 1, including input and output coupling means of said cavity, and a temperature-dependent reactive load increasing and decreasing in direct proportion to temperature and connected to said output coupling means.

8. In a cavity resonator including a closed metallic casing providing a resonant cavity, and input and output coupling means for said cavity, temperature stabilizing means comprising in combination:

(1) a first conductive reentrant member in contact with said casing and projecting into said cavity, and

(2) a second conductive reentrant member connected to said casing and having at least a partly hollow portion projecting into said cavity in. line with an enclosing said first member, to form a reentrant capacity by said members,

(3) the hollow portion of said second member being constituted by a plurality of spaced bi-metallic strips designed to be deflected away from said first member in proportion to increasing temperature and to be deflected towards said first member in proportion to decreasing temperature, respectively, and

(4) said members adjusted relative to one another, to provide a predetermined means reentrant capacity such as to cause the variations thereof by said strips to substantially compensate for the temperature-dependent resonant frequency changes of said cavity.

FOREIGN PATENTS 566,079 11/1958 Canada.

H. K. SAALBACH, Primary Examiner L. ALLAHUT, Assistant Examiner 

